ECG Criteria for Premature Atrial Contractions and Multifocal Atrial Tachycardia
For premature atrial contractions (PACs), look for a premature P wave with different morphology from sinus P waves that occurs before the next expected sinus beat, which may be conducted normally, with aberration, or blocked; for multifocal atrial tachycardia (MAT), the diagnostic triad requires ≥3 distinct P-wave morphologies, atrial rate >100 bpm, and an isoelectric baseline between P waves with irregular rhythm. 1, 2, 1
Premature Atrial Contractions (PACs)
Core ECG Features
P Wave Characteristics:
- A premature P wave appearing earlier than the next expected sinus P wave 2
- Different morphology and mean vector compared to sinus P waves 2
- The key is timing: in regular sinus rhythm at normal rate, any P wave occurring before the expected next P wave is a PAC 2
Conduction Patterns:
PACs can present in three ways 2:
- Conducted normally - followed by a normal QRS complex
- Conducted with aberration - followed by a wide QRS (can mimic premature ventricular contractions)
- Blocked (non-conducted) - P wave not followed by QRS complex
Critical Pitfall: Blocked Atrial Bigeminy
Watch for blocked PACs occurring in bigeminal sequence - this simulates sinus bradycardia and is easily missed 2. Carefully examine T waves for hidden blocked P waves, as this distinction is clinically important: blocked atrial bigeminy is benign while severe sinus bradycardia may indicate systemic illness 2.
Distinguishing PACs from PVCs
When you see premature beats with wide QRS complexes, search carefully for a premature P wave preceding the wide QRS before diagnosing both PACs and PVCs on the same strip 2. It's relatively uncommon for patients to have both simultaneously 2.
Multifocal Atrial Tachycardia (MAT)
Diagnostic Criteria (All Must Be Present)
The 2015 ACC/AHA/HRS guideline provides the definitive criteria 1:
- ≥3 distinct P-wave morphologies and/or patterns of atrial activation at different rates
- Atrial rate >100 bpm (some sources use >90 bpm 3)
- Irregular rhythm - the rhythm is ALWAYS irregular 1
- Isoelectric baseline between P waves - this distinguishes MAT from atrial fibrillation 3
Key Distinguishing Features
MAT vs Atrial Fibrillation:
- MAT has distinct P waves with isoelectric baseline between them 3
- AF has irregular atrial activity without distinct P waves 1
- Both have irregular R-R intervals, making differentiation challenging on physical exam alone 3
- A 12-lead ECG is mandatory to confirm the diagnosis 1
Variable Intervals:
- P-P intervals vary
- P-R intervals vary (may reflect decremental AV nodal conduction rather than just P-wave origin 1)
- R-R intervals vary 3
Clinical Context
MAT is most commonly seen in elderly patients with severe cardiopulmonary disease, particularly COPD, and is associated with hypoxia, hypokalemia, acidemia, and increased catecholamines 3, 4. The mechanism is likely triggered activity from increased intracellular calcium 3, 4.
Common Misdiagnosis
MAT is frequently mistaken for atrial fibrillation 3, 4, which can lead to inappropriate digoxin therapy (ineffective for MAT and potentially toxic in these predisposed patients) 4. When atrial fibrillation has a rapid ventricular response, the irregularity becomes less obvious and may be misdiagnosed as regular SVT 5.